An electric power steering apparatus that energizes a steering mechanism of a vehicle by using a rotational torque of a motor as a steering assist force (assist torque), applies a driving force of the motor as the steering assist force to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and the motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a pulse width modulation (PWM) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage command value Vref obtained by performing compensation and so on with respect to the current command value in a current control section. A steering angle sensor 14 is necessarily needed, and it may not be provided.
A controller area network (CAN) to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 51 is also possible to connect to the control unit 30, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 30 mainly comprises an MCU (including a CPU or the like), and general functions performed by programs within the MCU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from CAN 50) are inputted into a current command value calculating section 31 to calculate a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 which is a current desired-value of the current supplied to the motor 20 based on the steering torque Th and the vehicle speed Vel and by means of an assist map and so on. The current command value Iref1 is inputted into a current limiting section 33 via an addition section 32A, and a current command value Irefm of which the maximum is limited is inputted into a subtraction section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im that is fed back, is calculated at the subtraction section 32B. The deviation I is inputted into a PI-control section 35 for improving the characteristic of the steering operation. The voltage command value Vref that characteristic improvement is performed in the PI-control section 35, is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a drive section. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtraction section 32B. In general, the inverter 37 uses EFTs as driving elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensation section 34 is added to the addition section 32A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 to an addition section 344, further adds the result of addition performed to the addition section 344 and a convergence 341 in an addition section 345, and then outputs the result of addition performed in the addition section 345 as the compensation signal CM.
In a case that the motor 20 is a 3-phase brushless motor, details of the PWM-control section 36 and the inverter 37 is a configuration such as shown in FIG. 3. That is, the PWM-control section 36 comprises a duty calculating section 36A that calculates, in synchronism with a PWM-carrier, PWM-duty command values D1 to D6 of 3-phases according to a predetermined expression based on the voltage command value Vref and a gate driving section 36B that drives each gate of FETs with the PWM-duty command values D1 to D6 and switches ON/OFF after dead time compensations. The inverter 37 comprises a three-phase bridge having upper and lower arms comprised of the upper FET1 and the lower FET4 of U-phase, upper and lower arms comprised of the upper FET2 and the lower FET5 of V-phase, and upper and lower arms comprised of the upper FET3 and the lower FET6 of W-phase, and drives the motor 20 by being switched ON/OFF based on the PWM-duty command values D1 to D6.
In these electric power steering apparatus, it is necessary to detect each phase current of the motor 20 and to feedback them, and as one of request items for a compact unit, lighting and cost down, unitizing (1-shunt type current detection circuit) of the current detection circuit is proposed. The 1-shunt type current detection circuit is known as the unitizing of the current detection circuit, for example, it is disclosed in Japanese Published Unexamined Patent Application No. 2009-131064 A (Patent Document 1) and WO 2013/077241 (Patent Document 2).
On the other hand, in the electric power steering apparatus, there are also vehicle types which are desired a multiplex system in order to raise a reliability of the apparatus as well as for the purpose of fail-safe and a stability even if the cost-up rises. This is the same with the current detection system to detect each phase current of the motor.